1. Field of the Invention
The present invention relates to a branching filter and, in particular, to a branching filter including a ladder-type elastic wave filter unit connected between an antenna terminal and a transmission signal terminal and a longitudinally coupled resonator-type elastic wave filter unit connected between the antenna terminal and first and second balanced reception signal terminals.
2. Description of the Related Art
In recent years, elastic wave branching filters using elastic waves, such as surface acoustic waves or boundary acoustic waves, have been widely used as a branching filter for branching transmission signals to be transmitted through an antenna and reception signals received through the antenna.
For example, Japanese Unexamined Patent Application Publication No. 2007-189390 describes a duplexer using a highly power-resistant, ladder-type elastic wave filter as a transmission filter and using a longitudinally coupled resonator-type elastic wave filter with the balanced-unbalanced transforming function as a reception filter.
Since a duplexer as described in Japanese Unexamined Patent Application Publication No. 2007-189390 is miniaturized, a change in the disposition of the ladder-type elastic wave filter or longitudinally coupled resonator-type elastic wave filter may deteriorate the isolation characteristic between a transmission signal terminal to which the ladder-type elastic wave filter is connected and first and second reception signal terminals to which the longitudinally coupled resonator-type elastic wave filter is connected.